I've been fortunate enough to volunteer with VESL for the past two years as a student coordinator, based in Thailand for a large part of both Summers. I've worked with other charities before but none so far have compared to VESL.
It's run by a team of extremely dedicated past volunteers and what I think is so great about this charity is that you really don't need any experience. I've seen people go with no teaching experience at all but they had the right attitude and had the time of their lives. The programmes benefit both the pupils in Sri Lanka, India and Thailand but also the volunteers.
I think if I had to choose one highlight it would be the day that myself and my team taught a group of secondary school teachers. They were so kind and fun and willing to learn, we spent hours singing songs and having fun - then after that they took us all out for lunch and taught us some thai. Another highlight would be just how great the kids are. So disciplined and so eager, it was brilliant to leave them knowing that they had significantly better English skills than before our arrival - and that hopefully they would be inspired to continue on this path of learning.
I would reccomend VESL to absolutely any one, as long as you like meeting new people, aren't afraid of a bit of hard work and as long as you're ready for a life changing experience :)